This invention relates to inspection of process fluids used in the manufacture of semiconductor components and other microelectronic devices and specifically relates to particulate inspection of liquids by interferometric light measurements. More particularly, the phase shift of light travelling through a local index perturbation or object in the fluid such as a bubble or particle, is detected and analyzed in order to determine the characteristics and size of the object.
Contamination control in the manufacture of semiconductor components and other microelectronic devices is becoming a process variable. Particulate contamination causes more than half of the yield loss in volume semiconductor manufacturing. A substantial amount of this loss is due to the chemicals, such as solvents, acids and bases and process gases, that come into contact with the wafers. The contaminant concentration in such fluids is typically more than three orders of magnitude greater than that present in clean room air and six orders of magnitude greater than that present in the next generation of clean rooms.
Presently used monitoring techniques for examining process fluids, both liquids and gases, have several major deficiencies that are overcome by the present invention. Currently available devices are unable to discriminate between gas bubbles and particles in a process liquid. These devices lose sensitivity for detecting particles having indices of refraction close to that of the fluid. Also, the inspected volume throughput in the available devices demonstrates a precipitous drop as the particle size threshold is reduced. Moreover, these devices have not demonstrated extendability to the one-tenth groundrule limit for detecting particles. The one-tenth groundrule limit refers to the case in which devices are manufactured, for example, in the one micron range, contaminants must be detectable to one-tenth of the range, or 0.1 micron size.
It is known to detect particles by measuring light scattered from the particles. This technique is effective for particle sizes having a circumference as small as the wavelength of the light, at which point the particles exhibit the characteristic Rayleigh scatter cross section that varies as the sixth power of the particle size. Small particles therefore become difficult to measure. Improving particle size sensitivity by a factor of two requires improving the scattered light detection sensitivity by a factor of 64.
An alternative method for improving particle size sensitivity is to consider the particle as a phase object, and measure the effect of the particle on a wavefront. Placing the particle in one arm of an interferometer achieves the desired measurement. Another technique, the preferred method, is to use bright field analysis as will be described hereinafter. Dark field analysis is another alternative technique for measuring the phase shift of the scattered light.